A.c. induction motors find ubiquitous application in major appliances, refrigerators, air-conditioners, and other machines of all sorts. Induction motors are cheap and simple to manufacture, and have an enviable record for long-term reliability without attention. Induction motors are relatively efficient electrically, when fully loaded. When lightly loaded, they also are notorious for wasting considerable amounts of electricity by consuming far more electrical power than what they are called upon to deliver as operating torque from their output shaft. It is this later rather troublesome shortcoming of common induction motors which needs improvement, and it is believed that this invention now can offer considerable relief.
To give some scope to the impact which induction motors have on society, one may consider that "125 million household refrigerators and freezers in operation today require the electricity from 30 standard (large sized) 1,000-megawatt power plants." ("Scientific American", Vol. 258, No. 4, April 1988, Page 56, in an article `Energy-efficient Buildings` by Arthur Rosenfeld and David Hafemeister.) Virtually all such refrigerators use induction motors. Considerable waste occurs because refrigerators do operate with varying mechanical load demands, while the typical hermetically sealed compressor assembly contains an induction motor which is sized to handle the worst case, ableit `normal`, compression load demand imposed upon it without frequent stalling. The result is simple: much of the time the motor is operating at less than full load and wasting a considerable amount of energy. Merely improving the dynamic operating efficiency of such a motor a mere ten-percent or so may result in the power saving equivalent to that afforded by 3 of the large 1,000 megawatt power plants, which are said to cost several billion dollars to construct. Some perspective of what this means can be obtained by considering an article which appeared in "New England Business" magazine, May 2, 1988, on pages 39-40 wherein the New England Power Pool (an organization which represents the region's utilities) estimates that by 1995 about 4,000-megawatts of additional generating capacity (on top of the present 23,000-megawatts of current capacity) would be needed just to keep up with demand growth. You also need to keep in mind that the highly controversial New Hampshire `Seabrook Unit I` and Plymouth, Mass. `Pilgram` nuclear power generating facitlities produce a combined power of only 1,820-megawatts: far less than what might be conserved through better induction motor operating efficiencies!Hence, improved efficiency in motor operation for refrigerators, air-conditioners, and other machines could down-scale the demand growth and alleviate some of the pressures which now exist in getting additional capacity on-line. Needless to say, greater improvements in motor efficiency can afford even more spectacular economic savings in power plant needs and reductions in attendant `wasteful` consumption of non-renewable fuel resources. Such further improvement in induction motor operating efficiency is precisely what is brought about by my instant invention.
Modern induction motors are often designed to operate with very high magnetic field flux densities in the stator structure. The result is near-saturation of the core material, with high eddy current losses. In addition, the windings may be designed to operate with high current densities that results in considerable heating due to winding resistance losses. Such winding losses are further aggravated in many cheaply designed appliance motors through the use of aluminum wire in lieu of the better and generally more efficient copper wire windings. Motor design my be dictated by competitive market conditions, resulting in agressive cost cutting. Cheap designs commonly translate into producing motors having high operating levels and the result may be a motor which operates with reasonable efficiency under full load, while under light load it is a wasteful of considerable energy. High temperature rise in a lightly loaded (or unloaded) motor is a sure sign indicating poor electrical operating efficiency. Modern motors operate very hot under all conditions of loading, which translates into poor overall performance efficiency when a widely varying load is being driven by the motor.
In earlier U.S. Pat. Nos. 4,052,648 and 4,266,177 Frank Nola describes how the a.c. pwoer fed to an induction motor might be controlled and therefore bring about some improvement in electrical efficiency. While he does measure the power factor of the operating motor and therefrom determines various control values for regulating the total power input of the motor run winding set by conventional phase-angle controlled firing of a triac, he greatly reduces and in some cases may negate any purported improvement by virtue of the severe a.c. power waveform harmonic distortion which his system reflects into the electric utility system. Nola also describes apparatus which is fraught with possibilities for maladjustment and drift, and wherein the correct operating points are not pre-established by design but rather they must be somewhat tailored to each unit which might be manufactured, resulting in a relatively labor-intensive and costly product. Column 3, lines 40-47 and column 6, lines 50-66 of U.S. Pat. No. 4,266,177 particularly describes the kind of twiddling that is needed to set the device's operating points relative with any particular motor's observed performance.
In yet another U.S. Pat. No. 4,533,857, Ten-Ho Chang et al show a motor controller which in effect measures the apparent current drawn by an induction motor and therefrom develops a variously retarded phase-angle control signal which fires a triac and thus modulates the total power flow to the motor. Like Nola, Chang et al shows the turn-ON of the full motor running current at some delayed point during each a.c. half-cycle and of course such an approach is fraught with severe harmonic distortion of the a.c. power flow (as reflected into the a.c. power lines), accompanied by resulting inefficiencies that may exceed any gain which could otherwise be obtained from use of the controller. Chang also does not recognize nor allow for the common characteristic of cheap induction motors wherein the lightly loaded (or even unloaded) apparent motor current may be only a little less than what full load motor current is. Although the motor current is lagging by perhaps 60 degrees or more, the actual measurable current which develops across the current transformer (Tr-2 in Chang's teaching) will be quite nearly the same as what develops under full motor load, when the motor current might lag by 30-degrees or less. A typical appliance motor, such as the General Electric type 5KH46JR15S has been found to draw about 7.9 amperes under full load, and yet continue to draw nearly 7 amperes of apparent current under NO-LOAD. Power factor also varied from about 80-85% under FULL-LOAD, to about 20-30% under NO-LOAD. This of course means that little change in current occurs and the circuit of Chang would operate ineffectively because slight changes in a.c. line voltage bring about more substantial changes in motor current than what changes in motor load produce. Chang is silent regarding compensation of apparent motor current changes which merely relate to commonplace a.c. line voltage fluctuations.
Noise, in the form of hum and buzz, are byproducts of stressful motor operating conditions. Magnetostrictive effects tend to produce substantial noise in the motor's structure, paticularly when stressed with the strong and abrupt changes in flux brought about by phase-delayed thyristor power control. These abrupt changes in induction fields can also set up other parasitic vibrations which, aside from being audibly annoying, can lead to premature structural fatigue of the motor's components (such as a vibrating loop of wire which eventually breaks off). Refrigerators and, to a lesser extent, air conditioners are frequently annoying sources of audible noise, albeit not particularly high in the sense of loudness on the commonly cited decibel scale for noise sources. Load related modulation of power flow to such motors may therefore serve to substantially abate such undesirable noise and result in a more acceptable product.